The invention relates generally to laser shock peening processes and systems and, more particularly, to a system and method for process monitoring and quality assurance for laser shock peening.
Laser shock peening or laser shock processing, as it is also referred to, is a process for producing a region of deep compressive residual stresses imparted by directing a laser beam on a surface area of a workpiece. Laser shock peening typically uses multiple radiation pulses from high power pulsed lasers to produce shock waves in a workpiece similar to methods disclosed in U.S. Pat. No. 3,850,698, entitled “Altering Material Properties”; U.S. Pat. No. 4,401,477, entitled “Laser Shock Processing”; and U.S. Pat. No. 5,131,957, entitled “Material Properties”. Laser peening has been utilized to create a compressively stressed protection layer at the outer surface of a workpiece which is known to considerably increase the resistance of the workpiece to fatigue failure as disclosed in U.S. Pat. No. 4,937,421, entitled “Laser Peening System and Method”. The methods typically employ a curtain of water flowing over the workpiece or some other method to provide a confining medium to confine and redirect the process generated shock waves into the bulk of the material of a component being laser shock peened to create the beneficial compressive residual stresses. Other techniques to confine and redirect the shock waves that do not use water have also been developed.
Laser shock peening is being developed for many applications in the gas turbine engine field, some of which are disclosed in the following U.S. Pat. No. 5,756,965 entitled “ON THE FLY LASER SHOCK PEENING”; U.S. Pat. No. 5,591,009, entitled “Laser shock peened gas turbine engine fan blade edges”; U.S. Pat. No. 5,569,018, entitled “Technique to prevent or divert cracks”; U.S. Pat. No. 5,531,570, entitled “Distortion control for laser shock peened gas turbine engine compressor blade edges”; U.S. Pat. No. 5,492,447, entitled “Laser shock peened rotor components for turbomachinery”; U.S. Pat. No. 5,674,329, entitled “Adhesive tape covered laser shock peening”; and U.S. Pat. No. 5,674,328, entitled “Dry tape covered laser shock peening”. Successful laser shock peening requires efficient coupling of the laser generated shock wave into the workpiece. If the shock wave is not well coupled into the part being peened, then a level of compressive residual stress is reduced and the desired effect of the laser shock processing is detrimentally affected. Accordingly, any laser shock processing technique would benefit from efficient quality assurance testing during production runs using laser shock peening.
One laser shock peening quality assurance technique previously used is to subject a small sample of processed parts to high cycle fatigue (HCF) testing to verify the desired improvement in fatigue life required from the laser shock processing and notched in a laser shock peened area before testing. This method is destructive of the test piece, fairly expensive and time consuming to carry out, and significantly slows production and the process of qualifying laser shock peened components. An improved quality assurance method of measurement and control of laser shock peening that is accurate, non-destructive, robust in a manufacturing environment, and economically practical is highly desirable. It is also desirable to have a real time non-destructive process monitoring method that may be practically implemented on each workpiece instead of a small sampling of workpieces. Laser shock peening is a process that, as any production technique, involves machinery and is time consuming and expensive. The use of a real time non-destructive evaluation method allows process deviations to be discovered during a production run and corrected immediately.
Therefore, it is desirable to employ a real time process monitoring technique during the laser shock peening process that can address one or more of the aforementioned issues.